The present invention relates to a noise immune printed circuit particularly for use in tape recorders and record players.
Playback amplifiers used in tape recorders and record players are usually provided with an equalizer to emphasize lower frequency components to compensate for particular record-playback characteristics. The input circuit of the playback amplifier comprises a first circuit section which couples the base of a transistor to a first terminal of the playback head, and a second circuit section which couples the emitter of the transistor to the second terminal of the head. The collector of the transistor is coupled by way of capacitors to the equalizer. A loop is thus formed from the first and second circuit sections of the amplifier input circuit. In the presence of an external magnetic flux generated by a power supply unit or the like, a current will be induced by the magnetic flux in the loop circuit and fed as an induced noise to the amplifier. Since the frequency of the noise is low, the induced noise is considerably amplified by the equalizer. To suppress the noise the following points have usually been taken into consideration:
(1) To place the input circuit and the noise generating source as far apart from each other; PA0 (2) To design a circuit pattern that minimizes the area of the loop; and PA0 (3) To provide a magnetic shield over the loop.
However, difficulties have been encountered to meet these requirements simultaneously.